The most familiar air moving mechanism is the simple axial fan, which is the stationary equivalent of an airplane propeller. Whether used for residential cooling or automotive radiator cooling, it simply pulls air axially straight through it. Less familiar is the so called centrifugal blower, which finds common usage in vehicle HVAC systems. A centrifugal blower has a generally cylindrical impeller rotating in one direction that pulls air in along its central axis as it rotates, but then forces it radially outwardly, turning it ninety degrees, in effect. A scroll shaped blower housing surrounding the impeller collects and confines the expelled air and sends it through a tangential outlet to the rest of the HVAC system.
The basic cylindrical impeller includes a central hub, often dome shaped, through which a motor drive shaft is attached, and a flat, annular outer rim. Extending upwardly from the hub rim are an evenly spaced series of identical blades, which are parallel to the central axis. While the blades have a radial component, when viewed axially, they seldom lie right on radial planes, like a simple paddle wheel, but are generally sloped away from a radial plane, in one direction or the other. That overall blade slope or incline may run either toward the direction of rotation, called forwardly curved or inclined, or away from the direction of rotation, called rearwardly curved or inclined. "Inclined" is a more useful term to describe the blade angle, since the blades are seldom flat, either, and may be concave, convex, or an S shaped combination of the two. This concavity-convexity may be thought of as a "curve" of the blade, as distinct from its overall slope or inclination relative to the direction of rotation. In general, rearwardly inclined blades are more nearly flat than forwardly sloping blades, which are often distinctly concave or scoop shaped. There are known advantages and disadvantages to both types of blade. Rearwardly inclined blades are known to produce a pressure differential that is more static than dynamic, and are self limiting in their power demand at high impeller speeds. A disadvantage of rearwardly inclined fans is that it is more critical to preserve that high static pressure by limiting potential leak paths at the interface between the impeller and blower housing.
Another consideration with either type of impeller, but especially with rearwardly inclined blades, is ease of manufacture. In automotive applications, only molded plastic impellers with a minimum number of parts and assembly steps are cost effective. As will be recognized by those skilled in the molding art, the most cost effective way to mold plastic is by the so called axial draw technique, which uses only two molds that part along a natural axis of the component. It is possible to mold a centrifugal blower impeller's hub, hub rim, and blades all in one piece, with molds parting along the hub central axis. This is because the outer surfaces of the blade all run parallel to that central axis, with no undercuts. The bases of the blades are integrally molded to the hub rim, but the tips of the upstanding blades are unsupported and free. Without stiffening support of some sort, the blade tips and blades would twist and flex unacceptably in operation. It is possible to tie the blades together and stiffen them, to an extent, with a thin, circular end band that surrounds the outer edges of the blades, near the tips. Such an end band is everywhere larger in diameter than the outer edge of the hub rim. It therefore does not radially overlap with the central hub's rim at all, and may be molded integrally to and with the impeller, by the same two molds. An end band, which touches only the outer blade edge, can provide enough tip support for narrower, forwardly inclined blades, whose sharp concavity also gives them enough inherent stiffness to resist twisting in operation. However, rearwardly inclined blades have a much greater radial extent, as measured from inner to outer edge, and a consequently much wider unsupported tip. They also generally have much less "curvature" to help stiffen them. Consequently, a separate stiffening ring, axially opposed to and overlaying the hub rim, has generally been secured to the blade tips. The stiffening ring can also help to control pressure leakage between the impeller and the edge of the housing inlet. The stiffening ring has a radial width comparable to the hub rim to which it is axially opposed, overlapping it almost completely. It therefore cannot be molded integrally to the rest of the impeller, and has to be manufactured separately. It is generally staked to each separate blade tip, a time consuming and expensive assembly step.